U.S. patent number 11,143,953 [Application Number 16/361,076] was granted by the patent office on 2021-10-12 for protection of photomasks from 193nm radiation damage using thin coatings of ald al2o3.
This patent grant is currently assigned to International Business Machines Corporation. The grantee listed for this patent is International Business Machines Corporation. Invention is credited to Peter H. Bartlau, Thomas B. Faure, Supratik Guha, Edward W. Kiewra, Louis M. Kindt, Robert L. Sandstrom, Alfred Wagner.
United States Patent |
11,143,953 |
Sandstrom , et al. |
October 12, 2021 |
Protection of photomasks from 193nm radiation damage using thin
coatings of ALD Al2O3
Abstract
The invention relates to a method used in a photolithographic
process comprising depositing a film of Atomic Layered Deposition
(ALD) Al.sub.2O.sub.3 on a photomask, subjecting said film of
Al.sub.2O.sub.3 on the photomask to a plasma treatment and then
irradiating the deposited film of ALD Al.sub.2O.sub.3 on the coated
photomask at a wavelength of 193 nm.
Inventors: |
Sandstrom; Robert L. (Chestnut
Ridge, NY), Bartlau; Peter H. (Essex Junction, VT),
Faure; Thomas B. (Milton, VT), Guha; Supratik (Chicago,
IL), Kiewra; Edward W. (Underhill, VT), Kindt; Louis
M. (Underhill, VT), Wagner; Alfred (Brewster, NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
1000005863000 |
Appl.
No.: |
16/361,076 |
Filed: |
March 21, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20200301270 A1 |
Sep 24, 2020 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F
1/68 (20130101) |
Current International
Class: |
G03F
1/68 (20120101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0005739 |
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Dec 1979 |
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EP |
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0049799 |
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Nov 1982 |
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EP |
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Other References
Puurunen, Riika, (Jun. 15, 2005) "Surface Chemistry of Atomic Layer
Deposition: A case study for the trimediylaluminum/water process."
Journal of Applied Physices. 97 (12): 121301. doi
10.1063/1.1940727. ISSN 0021-8979. cited by applicant .
Miikkulainen, Ville; Leskaela, Marku; Ritala, Mikko; Puurunen,
Riikka L. (Jan. 14, 2013). "Crystallinity of Inorganic Films Grown
by Atomic Layer Deposition Overview and General Trenda," Journal of
Applied Physics. 113 (2) 021301. doi 10.1063/1.4757907. ISSN
0021-8979. cited by applicant .
"How Atomic Layer Deposition Works" Applied Materials.
https://www.youtube.com/watch?v=KOEsqZU1sts. cited by applicant
.
Hans-Jurgen Butt; Karlheinz Graf; Michale Kappl (2013). Physics and
Chemistry of Interfaces (Third, Revised Ed.). ISBN
978-3-527-41216-7. cited by applicant .
"2.3 Adsorption Kinetics--The Rate of Adsorption."
www.chem.qmul.ac.uk. cited by applicant .
George, S. M. (2010) "Atomic Layer Deposition: An overviw." Chem.
Rev. 110: 111-131. cited by applicant .
A. Ahnd, Semicond. Int. 26, 46-51, 2003. cited by applicant .
Detlev Ristau et al., "Ion beam sputter coatings for laser
technology," Proc. of SPIE, vol. 5963, 596313, 2005, abstract and
excerpt 4 pages total. cited by applicant.
|
Primary Examiner: Fraser; Stewart A
Attorney, Agent or Firm: Morris; Daniel Otterstedt, Wallace
& Kammer, LLP
Claims
What we claim and desire to protect by Letters Patent is:
1. A method in a lithographic process comprising: depositing a film
of Al.sub.2O.sub.3 on a photomask via an atomic layer deposition
method (ALD); subjecting said film of ALD deposited Al.sub.2O.sub.3
on said photomask to a plasma treatment; and irradiating said film
of ALD deposited Al.sub.2O.sub.3 on said photomask at a wavelength
of 193 nm.
2. The method defined in claim 1, wherein said photomask is formed
of one of chromium on glass (COG) and opaque molybdenum silicide on
glass (OMOG).
3. The method defined in claim 2, wherein said plasma treatment is
oxygen ashing.
4. The method defined in claim 3, wherein the thickness of said
Al.sub.2O.sub.3 deposited via ALD on said photomask is between
about 0.5 nm and 6 nm.
5. The method defined in claim 4, wherein said photomask is COG and
said thickness of said Al.sub.2O.sub.3 deposited via ALD on said
photomask is 5 nm thick.
6. The method defined in claim 5 wherein said 193 nm irradiation is
conducted in a dry air environment.
7. The method defined in claim 5, wherein said 193 nm irradiation
is conducted in a 40% humidity environment.
8. The method defined in claim 4, wherein said photomask is OMOG
and said thickness of said Al.sub.2O.sub.3 deposited via ALD on
said photomask is 1.1 nm thick.
9. The method defined in claim 8 wherein said 193 nm irradiation is
conducted in a dry air environment.
10. The method defined in claim 8, wherein said 193 nm irradiation
is conducted in a 40% humidity environment.
11. A computer program for depositing a film of Al.sub.2O.sub.3 on
a photomask on a photomask via an atomic layer deposition method
(ALD), said computer program product comprising: a computer
readable storage medium having stored thereon: first program
instructions executable by a processing circuit device to cause
said device to deposit a film of Al.sub.2O.sub.3 on said photomask
via an atomic layer deposition (ALD) method; and second program
instructions executable by a processing circuit device to cause
subjecting said photomask with said film of ALD Al.sub.2O.sub.3
thereon to a plasma treatment; and third program instructions
executable by a processing circuit device to cause irradiation of
said film of AID Al.sub.2O.sub.3 on said photomask at a wavelength
of 1.93 nm.
12. The computer program product defined in claim 11, wherein said
photomask is selected from the group of chromium on glass (COG) and
opaque molybdenum silicide on glass (OMOG).
13. The computer program product defined in claim 12, wherein said
plasma treatment is oxygen ashing.
14. The computer program product defined in claim 13, wherein said
first program instructions cause said thickness of said ALD
Al.sub.2O.sub.3 deposited on said photomask is between 0.5 nm and 6
nm.
15. The computer program product defined in claim 14, wherein said
first program instructions cause said thickness of said
Al.sub.2O.sub.3 deposited by said ALD process on said photomask to
be 5 nm on COG and 1.1 nm on OMOG.
16. The computer program product defined in claim 15 wherein said
193 nm irradiation is conducted in a dry air environment.
17. The computer program product defined in claim 14 wherein said
193 nm irradiation is conducted in a 40% humidity environment.
18. A method in a lithographic process comprising: depositing a
film of Al.sub.2O.sub.3 on a photomask via an atomic layer
deposition method (ALD); subjecting said film of ALD deposited
Al.sub.2O.sub.3 on said photomask to a plasma treatment; and
irradiating said film of ALD deposited Al.sub.2O.sub.3 on said
photomask at a wavelength of 193 nm, said photomask is formed of
one of chromium on glass (COG) and opaque molybdenum silicide on
glass (OMOG).
19. The method defined in claim 18, wherein said plasma treatment
is oxygen ashing.
Description
BACKGROUND
The present invention relates to a photomask, the method of
producing same and the materials used in conjunction with
photo-lithographic processes.
It is known in the field of printed circuit fabrication or
microelectronic fabrication, to use a photomask in forming images
utilizing a photo resist method.
The photolithographic process uses a light sensitive polymer,
called a photoresist, which is exposed to radiation and developed
to form three-dimensional relief images on a silicon wafer layer
substrate. The ideal photoresist image has the exact shape of the
designed or intended pattern in the plane of the substrate, with
vertical walls through the thickness of the resist.
The exposure step modifies the chemistry of the photoresist so that
the exposed portion either dissolves in a so-called developer and
washes away--(Positive resist), or solidifies and remains on the
mask while all the unexposed resist washes away in the
developer--(Negative resist).
Thus, the final resist pattern is binary: parts of the substrate
are covered with resist while other parts are completely uncovered.
This binary pattern is needed for pattern transfer since the parts
of the substrate covered with resist will be protected from
etching, ion implantation, or other pattern transfer mechanism.
A photomask, in combination with a photoresist, plays an essential
role in the overall lithographic process. The general sequence of
processing steps for a typical photolithography process includes
the steps of substrate preparation, a photoresist spin coat,
prebake, exposure, post-exposure bake, develop. A resist strip is
the final operation in the lithographic process, after the resist
pattern has been transferred into the underlying layer, typically
by some sort of etch process. This sequence of steps is generally
performed on several tools linked together into a contiguous unit
called a lithographic cluster, controlled by a computer, processor,
or processing circuit.
After the prebaking step in the sequence of processing steps noted
above, the photoresist film is photosensitive and is ready to be
exposed to the optical light source. A photomask, which contains a
negative or positive replica of the desired pattern, depending on
whether a negative or a positive photoresist (PR) layer is exposed
to a UV light source such that the pattern is transferred to the
soft baked PR coated film and is exposed to a UV light source.
One type of photomask is a Chrome on Glass (COG) mask. COG
photomasks are generally chrome coated lithographic templates on
pure silica glass (or quartz) designed to optically transfer
patterns on to other substrates. The photomask is an essential
hardware component in photolithography or any pattern replication
method which uses optical means for transferring the structure. The
desired pattern is "drawn" on the photo mask itself. If a
topographic feature is generated on the photo resist layer, the
lateral dimension of the features will depend on the patterns
created on the mask, while the vertical dimension of the pattern
will depend on the thickness of the PR layer. The pattern
information on the mask is created in a drawing package and stored
in a database, reformatted and transferred to a "writer", which can
be a laser writer or e-beam writer.
The typical COG photomask if formed from a quartz glass piece which
is fully coated with chromium. Chromium is opaque to UV light and
does not allow any light to pass. Thus, when UV light is projected
through the COG mask, the chromium absorbs the light in the
non-etched opaque regions where the light passes through the quartz
blank in the etch regions, forming the desired pattern on the PR
coated film-chromium-coated glass piece is positioned on the PR
coated film. This allows the light to fall only over the areas of
the PR film following the contour of the patterns available on the
photomask.
Fabrication of the photo mask therefore requires preferential
removal of the chrome layer, which should correspond to the desired
pattern or structure on the photo resist surface.
Oddly a photoresist layer is used to fabricate a photo mask.
Normally, a positive photoresist is used while making a photomask,
which is coated on the chrome coated quartz substrate. Typically, a
laser beam or an electron beam is dispersed over this PR layer to
generate the desired pattern. Since a positive PR is used, the
exposed areas (areas over which the electron beam or the laser
light has traveled) will wash away during development and will
expose the chrome layer beneath it.
The exposed portion of the chromium layer is etched away by either
a plasma or wet etch process, resulting in areas through which
light can pass.
There was a time when scientists predicted the end of
photolithography as a viable method of making faster and cheaper
processors. Their thought was that it would not be possible to
produce a structure smaller than one micrometer. As technology
progressed, it required shorter wavelengths of light and more
adroit techniques to achieve smaller elements.
For example, processors made in the 1970s through earlier methods
used regular white light to produce processors on a scale of 10
micrometers. Currently, extreme ultra violet light is used for its
smaller wavelength.
One photomask in use today is a so called blank. The resist is
sensitive to electron beam bombardments and can be transferred into
the chrome layer via etch processes. The chrome represents opaque
areas on the photomask which are responsible for the casting of
shadow during exposure of the silicon wafers.
Besides the traditional glass substrate which is coated with a
chrome and a resist layer, i.e., chrome on glass mask (COG), there
are various other types of photomasks which enhance the optical
resolution of the structures. The central issue of COG masks is the
diffraction of the light on edges. The light will not only impact
in perpendicular direction but will be deflected into areas which
must not be exposed.
Another photomask is the attenuated phase shift mask (AttPSM;
called a half tone mask) which uses a patterned layer of molybdenum
doped silicon oxynitride (MoSi) which represents the structures of
the circuit. The MoSi has a thickness which causes a phase shift of
the transmitted light of 180.degree.. Thus, the phase shifted light
and the radiation which transmits through glass only interfere
destructively. In addition, the MoSi is dense (6% or 18% @ 193 nm
wavelength). On the one hand the light is attenuated and on the
other hand the light waves which are in opposite phase erase each
other almost completely, resulting in a higher contrast.
A chrome layer can be added to areas which are not used for
exposure to mask unused regions. These photomasks are named tritone
masks.
A basic principle behind the operation of a photoresist is the
change in solubility of the resist in a developer upon exposure to
light (or other types of exposing radiation).
Contact and proximity lithography are the simplest methods of
exposing a photoresist through a photomask master pattern.
Contact lithography offers high resolution (down to about the
wavelength of the radiation), but practical problems such as
photomask damage from this radiation bombardment and resulting low
yield make this process unusable in most production manufacturing
environments.
Proximity printing reduces photomask damage by keeping the mask a
set distance above the wafer (e.g., 20 .mu.m above the actual
surface). Unfortunately, the resolution limit is increased to
greater than 2 to 4 .mu.m, making proximity printing insufficient
for today's technology, by far the most common method of exposure
is projection printing.
Projection lithography derives its name from the fact that an image
of the photomask is projected onto the wafer. Projection
lithography became a viable alternative to contact/proximity
printing in the mid-1970s when the advent of computer-aided lens
design and improved optical materials allowed the production of
lens elements of sufficient quality to meet the requirements of the
semiconductor industry. In fact, these lenses have become so
perfect that lens defects, called aberrations, play only a small
role in determining the quality of the image. Such an optical
system is said to be diffraction-limited, since it is diffraction
effects and not lens aberrations which, for the most part,
determine the shape of the image.
An important feature that must be considered in a photolithographic
process is "Resolution." Resolution is the smallest feature that
can be printed with adequate control, and has two basic limits: the
smallest image that can be projected onto the wafer, and the
resolving capability of the photoresist to make use of that image.
From the projection imaging side, resolution is determined by the
wavelength of the imaging light (.lamda.) and the numerical
aperture (NA) of the projection lens according to the Rayleigh
criterion: R.apprxeq..lamda./NA
Lithography systems have progressed from blue wavelengths (436 nm)
to UV (365 nm) to deep-UV (248 nm) to today's mainstream
high-resolution wavelength of 193 nm.
In the meantime, projection tool numerical apertures have risen
from 0.16 for the first scanners to amazingly high 0.93 NA systems
today producing features well under 100 nm in size.
Before the exposure of the photoresist with an image of the mask
can begin, this image must be aligned with the previously defined
patterns on the wafer. This alignment, and the resulting overlay of
the two or more lithographic patterns, is critical since tighter
overlay control means circuit features can be packed closer
together. Closer packing of devices through better alignment and
overlay is nearly as critical as smaller devices through higher
resolution in the drive towards more functionality per chip.
Another important aspect of photoresist exposure is the standing
wave effect. Monochromatic light, when projected onto a wafer,
strikes the photoresist surface over a range of angles,
approximating plane waves. The aforementioned monochromatic light
travels down through the photoresist and, if the substrate is
reflective, is reflected back up through the resist. The incoming
and reflected light interfere to form a standing wave pattern of
high and low light intensity at different depths in the
photoresist. This pattern is replicated in the photoresist, causing
ridges in the sidewalls of the resist feature. As pattern
dimensions become smaller, these ridges can significantly affect
the quality of the feature. The interference that causes standing
waves also results in a phenomenon called swing curves, the
sinusoidal variation in linewidth with changing resist thickness.
These detrimental effects are best cured by coating the substrate
with a thin absorbing layer called a bottom antireflective coating
(BARC) that can reduce the reflectivity seen by the photoresist to
less than 1 percent.
Photomask materials of the type described above degrade when
exposed to 193 nm light in lithographic tools. As an example, MoSi
oxidizes and chromium-based materials undergo chromium migration.
In both cases the degradation results in an unacceptable change in
the size of the features on the mask. Once this degradation occurs,
the mask must be discarded. In addition, since the degradation
occurs continuously with use, the degradation is
progressive--resulting in changing performance of the mask during
its useful life and posing the problem of fabrication of faulty
integrated circuits.
SUMMARY
The present invention relates to a method used in a
photolithographic process comprising depositing a film of Atomic
Layered Deposition (ALD) Al.sub.2O.sub.3 on a photomask, subjecting
said film of Al.sub.2O.sub.3 on the photomask to a plasma treatment
and then irradiating the deposited film of ALD Al.sub.2O.sub.3 on
the coated photomask at a wavelength of 193 nm.
The photomask of the present invention may comprise any photomask
known in the art, but is preferably selected from the group
consisting of chromium on glass (COG) and opaque molybdenum
silicide on glass (OMOG). The photomask with the film layer of ALD
Al.sub.2O.sub.3 deposited thereon is subjected to a plasma
treatment, preferably, oxygen ashing.
The thickness of the ALD Al.sub.2O.sub.3 deposited on said
photomask is between 0.5 nm and 6 nm, preferably between about 1.1
nm and about 5 nm for optimum results.
The irradiation treatment of the coated photomask can be optimally
conducted in a dry air environment or in a 40% humidity
environment.
The invention embodies a very thin layer of protective material.
More specifically, an atomic layer deposition (ALD) of
Al.sub.2O.sub.3, is deposited over the surface of a patterned
photomask which serves to block induced chemical degradation
resulting from the exposure of the coated photomask to a 193 nm
wave-length. By virtue of the atomic layer deposition (ALD) of a
thin film of Al.sub.2O.sub.3 on a COG photomask or an OMOG
photomask, the aforementioned photomasks are protected from
degradation when exposed to the 193 nm wave length during the
course of a photolithographic process.
The invention further relates to a computer program product for
depositing an ALD film of Al.sub.2O.sub.3 on a photomask. The
computer program product comprises a computer readable storage
medium having stored thereon: a first program instructions
executable by a processing circuit device to cause the device to
deposit a film of ALD Al.sub.2O.sub.3 on said photomask using an
atomic deposition method.
The second program instruction which is executable by a processing
circuit device caused the device to subject the photomask with the
film of ALD Al.sub.2O.sub.3 thereon to a plasma treatment.
The third program instructions executable by the processing circuit
device causes irradiation of the film of ALD Al.sub.2O.sub.3 on
said photomask at a wavelength of 193 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an atomic layer deposition process wherein
a first precursor is added to a reaction chamber containing a
surface to be coated to form a layer.
FIG. 2 is a schematic of an atomic layer deposition process wherein
a second precursor is added to a reaction chamber to react with a
first precursor to create another layer on the surface.
FIG. 3 is a schematic of an atomic layer deposition process wherein
the second precursor reacts with the first precursor to form
another layer on the surface.
FIG. 4 is a schematic of an atomic layer deposition process wherein
the first precursor and the second cursor form a desired thickness
in the reaction chamber.
FIG. 5 is a SEM photo of COG with no ALD Al.sub.2O.sub.3 coating
after irradiation.
FIG. 6 is a SEM photo of COG with an ALD Al.sub.2O.sub.3 coating
after irradiation.
FIG. 7 is a SEM photo of OMOG with no ALD Al.sub.2O.sub.3 coating
after irradiation.
FIG. 8 is a SEM photo of OMOG with ALD Al.sub.2O.sub.3 coating
after irradiation.
FIG. 9 is a SEM photo of a COG sample with 5 nm ALD Al.sub.2O.sub.3
coating oxygen ashed.
FIG. 10 is a SEM photo of a COG sample with a 1.1 nm ALD
Al.sub.2O.sub.3 coating
FIG. 11 is a SEM photo of OMOG sample with a 1.1 nm ALD
Al.sub.2O.sub.3 coating showing overlaid images.
FIG. 12 is a SEM photo of COG sample with a 1.1 nm ALD
Al.sub.2O.sub.3 coating showing thin chrome field images.
FIG. 13 is a graph depicting optical transmission, as a function of
wavelength of fused silica with and without 5 nm of an ALD
Al.sub.2O.sub.3 coating.
FIG. 14 is a graph plotting reflectivity as a function of
wavelength of fused silica with and without 5 nm of an ALD
Al.sub.2O.sub.3 coating film.
FIG. 15 is a graph plotting transmission difference as a function
of wavelength of an Al.sub.2O.sub.3 coating on fused silica with
and without 5 nm of an Al.sub.2O.sub.3 coating reflecting a
.DELTA.T.
DETAILED DESCRIPTION
The present invention relates to photolithography, also termed
optical lithography or UV lithography, which is a process used in
microfabrication to pattern parts of a thin film on the bulk of a
substrate. It uses light to transfer a geometric pattern from a
photomask to a light-sensitive chemical "photoresist", often
referred to simply as a "resist," on the substrate.
More particularly, the present invention relates to a specific
article, i.e., a photomask that is used in conjunction with the
photolithography process. Photomasks used for optical lithography
contain the pattern of the integrated circuits. The basis is a so
called blank: a glass substrate which is coated with a chrome and a
resist layer. The resist is sensitive to electron beams and can be
transferred into the chrome layer via etch processes. The chrome
represents opaque areas on the photomask which are responsible for
the casting of shadow during exposure of the silicon wafers.
One suitable photomask is the quartz/glass (substrate) that has a
layer of chrome on one side. The chrome is covered with an AR
(anti-reflective) coating and a photosensitive resist. The
photomask substrate with chrome, AR, and resist is known as a blank
photomask.
Besides the traditional chrome on glass mask (COG) there are
various types of photomasks which enhance the optical resolution of
the structures. The central issue of COG masks is the diffraction
of the light on edges. Thus, the light will not only impact in
perpendicular direction but will be deflected into areas which must
not be exposed.
There are different means which reduce the intensity of the
diffracted light. The AttPSM uses a patterned layer of molybdenum
silicide (MoSi) such as the OMOG, as used in the present invention,
which represents the structures of the circuit. The molybdenum
silicide is dense (6% or 18% at 193 nm wavelength). On the one hand
the light is attenuated, and on the other hand, the light waves
which are in opposite phase erase each other almost completely,
this results in a higher contrast.
The present invention's use of the OMOG binary photomask material
that employs MoSi as the absorber layer provides sufficient optical
density to appear opaque at 193 nm wavelengths while simultaneously
being thin enough to reduce the EMF effects that previously plagued
high NA immersion lithography. Since the entire industry has
decades of experience using MoSi films, OMOG is a perfect solution
for advanced lithography.
The OMOG material has particular advantage as a result of the OMOG
material stack utilizing a MoSi absorber with a higher extinction
coefficient, which allows further reduction in the film thickness
while maintaining sufficient optical density. Improved pattern
fidelity and better resolution are achieved through the combination
of a super thin chrome hard-mask and thinner resist. The MoSi
absorber, with its high anisotropic etch behavior, helps deliver
superior CD uniformity in the final product. Furthermore, the low
film stress inherent to the OMOG film stack helps to improve
flatness and reduce process-induced pattern placement errors. Thus,
OMOG photomasks provide finer resolution, increased fidelity,
tighter CD uniformity and better registration.
Chrome less phase shift masks don't use opaque films. The phase
shift is achieved by trenches which are directly etched into the
glass substrate. The manufacturing of these masks is difficult,
since the etch approach has to be stopped in the middle of the
glass. In contrast to etch processes where one layer is completely
etched till the layer beneath is reached--which causes changes in
the etch plasma, so that one knows when the process is finished--,
there is no indication when the exact depth in the substrate is
reached.
The alternating phase shift mask also uses trenches which are
etched into the glass substrate alternating to non-etched areas. In
addition, there are areas which are covered with a chrome layer to
decrease the intensity of radiation in these regions. However,
there are regions with an undefined phase shift, so that one has to
exposure twice with different masks. One mask contains the
structures which run in x-direction, while the second mask contains
the patterns which are orientated in y-direction.
The evolution of lithography wavelength corresponds to different
light sources. In lithography, wavelengths below 300 nm are called
"deep ultraviolet (DUV) light". In order to achieve the most
efficient way to achieve higher resolution, the present invention
uses a light source having a 193 nm wavelength. Moving to
wavelengths shorter than 193 nm presents problems.
The ability to project a clear image of a small feature onto the
wafer is limited by the wavelength of the light that is used, and
the ability of the reduction lens system to capture enough
diffraction orders from the illuminated mask.
Both new and recycled bare photomasks may have a thin invisible
layer of organic contamination on the Cr surface. This organic
layer will sometimes cause adhesion problems between the Cr and the
photoresist later in the process. It also may interfere with the Cr
etch process after photomask imaging. Whatever contamination exists
can be removed with a short, but aggressive oxygen plasma treatment
(ashing) before starting the photomask process. Plasma ashing as
used in the present invention, removes the photoresist from an
etched wafer. Using a plasma source, a monatomic reactive species
is generated. Oxygen or fluorine are the most common reactive
species and are used in accordance with the present invention. The
reactive species combines with the photoresist to form ash which is
removed with a vacuum pump.
Atomic layer deposition (ALD) is a thin-film deposition technique
based on the sequential use of a gas phase chemical process. ALD is
considered a subclass of chemical vapor deposition. In the ALD
thin-film deposition method a film grows on a substrate by exposing
its surface to alternate gaseous species, typically referred to as
precursors. These precursors react with the surface of a material
one at a time in a sequential, self-limiting, manner. Through the
repeated exposure to separate precursors, a thin film is slowly
deposited.
In contrast to chemical vapor deposition, the precursors are never
present simultaneously in the reactor, but they are inserted as a
series of sequential, non-overlapping pulses. In each of these
pulses the precursor molecules react with the surface in a
self-limiting way, so that the reaction terminates once all the
reactive sites on the surface are consumed. Consequently, the
maximum amount of material deposited on the surface after a single
exposure to all of the precursors (a so-called ALD cycle) is
determined by the nature of the precursor-surface interaction.[1]
By varying the number of cycles it is possible to grow materials
uniformly and with high precision on arbitrarily complex and large
substrates.
ALD is considered a one deposition method for producing very thin,
conformal film allowing control of the thickness and composition of
the films possible at the atomic level.
In the ideal ALD process one obtains extremely `flat` surface
behavior which is critical to focus on in photolithography
radiation to keep surface in focus for resolution purposes and this
is needed at the 193 nm wavelength.
In the ALD method used in the present invention, the thickness of
the Al.sub.2O.sub.3 coating is between about 0.5 nm and about 6 nm,
preferably between about 1.1 nm and 5 nm.
In contrast to chemical vapor deposition (CVD), the precursors are
never present simultaneously in the reactor, but they are inserted
as a series of sequential, non-overlapping pulses. In each of these
pulses the precursor molecules react with the surface in a
self-limiting way, so that the reaction terminates once all the
reactive sites on the surface are consumed. Consequently, the
maximum amount of material deposited on the surface after a single
exposure to all of the precursors (a so-called ALD cycle) is
determined by the nature of the precursor-surface interaction.
In a prototypical ALD process, a substrate is exposed to two
reactants A and B (See FIGS. 1 and 2) in a sequential,
non-overlapping way. In contrast to other techniques such as
chemical vapor deposition (CVD), where thin-film growth proceeds on
a steady-state fashion, in ALD each reactant reacts with a surface
in a self-limited way: the reactant molecules can react only with a
finite number of reactive sites on the surface. Once all those
sites have been consumed in the reactor, the growth stops. The
remaining reactant molecules are flushed away and only then is
reactant B charged into the reactor. By alternating exposures of A
and B a thin film is deposited. Consequently, when describing the
instant ALD process, one refers to both dose times (the time a
surface is being exposed to a precursor) and purge times (the time
left in between doses for the precursor to evacuate the chamber)
for each precursor. The dose-purge-dose-purge sequence of a binary
ALD process constitutes an ALD cycle. Also, rather than using the
concept of growth rate, ALD processes are described in terms of
their growth per cycle. In the ALD process, enough time is allowed
in each reaction step so that a full adsorption density can be
achieved. When this happens, the process has reached saturation.
This time will depend on two key factors: the precursor pressure,
and the sticking probability.
A basic schematic of the atomic layer deposition process is
depicted in FIGS. 1 to 4.
In FIG. 1, precursor 1 (represented by the solid circles) is added
to the reaction chamber containing the material surface to be
coated ALD. After precursor 1 has adsorbed on the surface, any
excess is removed from the reaction chamber. In FIG. 2, precursor 2
(represented by the open circles) is added and reacts with
precursor 1 to create another layer on substrate surface 3. In FIG.
3, precursor 2 is then cleared from the reaction chamber and this
process is repeated until a desired thickness is achieved and the
resulting product resembles the embodiment depicted in FIG. 4.
Substrate 1 is conveniently MoSi (OMOG) and Chrome on Glass
(COG).
The rate of adsorption per unit of surface area can be expressed
as: R adsorption=S*F
Where R is the rate of `adsorption`, S is the `sticking probability
and F is the `incident molar flux`.
A key characteristic of ALD is that S will change (ds/dt) with
time, wherein, as more molecules have reacted with the surface, the
sticking probability limit will approach zero once saturation is
reached.
The synthesis of Al.sub.2O.sub.3 from trimethylaluminum (TMA) and
water is one of the convenient sources of Al.sub.2O.sub.3 for use
in the present invention. The self-limited growth of
Al.sub.2O.sub.3 used in the method can be achieved in a wide range
of temperatures ranging from room temperature to more than
300.degree. C.
During the TMA exposure, TMA dissociatively chemisorbs on the
substrate surface and any remaining TMA is pumped out of the
chamber. The dissociative chemisorption of TMA leaves a surface
covered with AlCH.sub.3. The surface is then exposed to H.sub.2O
vapor, which reacts with the surface --CH.sub.3 forming CH.sub.4 as
a reaction byproduct and resulting on a hydroxylated
Al.sub.2O.sub.3 surface.
The atomic layer deposition transport mechanism is shown as:
##STR00001##
By varying the number of cycles, it is possible to grow materials
very uniformly and with high precision (resolution) on arbitrarily
simple, complex and large substrates.
ALD is one deposition method for producing very thin, conformal
films with control of the thickness and composition of the films
possible at the atomic level.
EXAMPLE
The following procedures were conducted to establish the
superiority of the use of Al.sub.2O.sub.3 when applied by atomic
layer deposition on patterned photomasks.
Two samples each of patterned Cr (COG) and MoSi (OMOG) photomasks
were subjected to oxygen plasma ashing, using the plasma generating
process described above.
Using atomic layer deposition (ALD), 5 nm of Al.sub.2O.sub.3 were
deposited on samples of COG and OMOG. The 5 nm COG and OMOG exposed
samples were exposed to 193 nm irradiation in dry air, followed by
humid air irradiation. Photos of the 5 nm COG and OMOG samples were
taken using a scanning electron microscope (SEM) producing images
of the 5 nm samples resulting from the interactions of a focused
beam of electrons with atoms at various depths within the samples.
The resulting photos were compared with SEM photos taken of COG and
OMOG samples containing no Al.sub.2O.sub.3 deposited thereon.
Meaningful information is obtained with respect to the surface
topography and composition of the COG and OMOG samples by using the
SEM photos that were taken of treated samples and non-treated
samples that produced images of the respective samples by scanning
their respective surfaces.
The SEM photos provided herein in FIGS. 5-12, serve to illustrate
difference in kind, rather than difference in degree as to the
effectiveness of the present invention embodiments by comparing the
sample possessing an Al.sub.2O.sub.3 coating layer applied by ALD
with a sample possessing with no Al.sub.2O.sub.3 coating.
FIG. 5 is a SEM photo of a COG sample that contains no
Al.sub.2O.sub.3 after irradiation that depicts the AR Cr layer and
the thin Cr layer. sample that was exposed to 193 nm irradiation in
dry air followed by 40% humid air environment.
The sample depicted in FIG. 5 did not have an ALD Al.sub.2O.sub.3
layer contacting its surface. The SEM photo shows the substantial
degradation of the sample containing an antireflective coating (AR)
Cr layer and a thin Cr layer after irradiation.
FIG. 6 is a SEM photo of a COG sample that contains 5 nm of
Al.sub.2O.sub.3 after irradiation. The SEM photos taken of the
sample depicted in FIG. 2, shows no degradation of the surface of
the sample as compared to the AR Cr layer and thin Cr layer.
The OMOG samples of FIGS. 7 and 8 have an ALD deposited 5 nm
Al.sub.2O.sub.3 film on their front side, and each was subject to
irradiation at 193 nm in dry air and in 40% humid air
environment.
FIG. 7 is a photo of a sample of OMOG containing no Al.sub.2O.sub.3
that shows significant change after irradiation.
FIG. 8 is a photo of an OMOG sample containing an ALD 5 nm
Al.sub.2O.sub.3 coating depicting no CD change after irradiation at
193 nm.
In a variant of this process, the 5 nm ALD deposited
Al.sub.2O.sub.3 coated sample of FIG. 2 was oxygen ashed, and
immediately exposed to 193 nm irradiation concurrent with its
exposure to an environment containing 40% humid air.
FIG. 9 is a SEM photo depicting a COG sample with an ALD deposited
5 nm Al.sub.2O.sub.3 coating. The sample was subjected to oxygen
plasma ashing and immediately irradiated in a 40% humid air
environment. The sample shows no evidence of any effect of
irradiation after oxygen ashing. The SEM photo of FIG. 9, indicates
that there is no evidence of any effect (CD change) of irradiation
damage.
FIG. 10 shows a SEM photo of a COG sample with a 1.1 nm
Al.sub.2O.sub.3 coating that had been subjected to oxygen plasma
ashing and immediately irradiated at 193 nm in humid air. The
Al.sub.2O.sub.3 coating was applied by atomic layer deposition on
patterned photomasks of the sample. The SEM photo of FIG. 10 shows
there is barely a noticeable effect decorating feature edge.
FIG. 11 contains SEM separate photos, the first of which is a top
view of an OMOG 1.1 nm ALD deposited Al.sub.2O.sub.3 coated sample
having a front side that was oxygen ashed followed by irradiation
at 193 nm in humid air. irradiation. The SEM photo cross-sectional
view at the bottom of FIG. 11, of the same sample shown in the top
view. In FIG. 11, in each instance, the overlayered images of
irradiated and unirradiated sites show no CD change.
FIG. 12 is a SEM photo depicting a COG sample front side with a 1.1
Al.sub.2O.sub.3 coating that was exposed to 193 nm irradiation in a
40% humid air environment. The sample in FIG. 12 has two areas
detailed as "thin Cr field" and "Cr features." The vigorous oxygen
ash procedure may have opened up holes in the 1.1 nm
Al.sub.2O.sub.3 film in the "thin Cr field." There is a small
degradation effect at the edges of the Cr features, but no effects
within the Cr features area.
The SEM photos were evaluated and the results comparing the optical
transmission and reflectivity of bare fused silica were measured
and compared with fused silica coated with 5 nm ALD
Al.sub.2O.sub.3. There is a significant CD change in the
Al.sub.2O.sub.3 without the ALD processing, and in the OMOG sample
depicted in FIG. 8, the 5 nm ALD Al.sub.2O.sub.3 coating on its
surface, as the SEM photo shows, there is no CD change after
irradiation.
Using the same process described above for preparing samples
containing a 5 nm ALD Al.sub.2O.sub.3 coating, samples containing
1.1 nm Al.sub.2O.sub.3 were subjected to oxygen plasma ashing and
exposed immediately to 193 nm irradiation in 40% humid air
irradiation. The SEM photos were taken that produced images of the
1.1 nm samples by scanning their respective surfaces. No CD change
was noticed here, and the images were `sharp` and clear and also
all line definition was clear.
FIG. 12 is a SEM photo of a GOG sample having a 1.1 nm ALD
deposited Al.sub.2O.sub.3 coating, The sample was O.sub.2 ashed,
followed by irradiation at 193 nm in humid air. The vigorous
O.sub.2 may have opened up the pinholes in the 1.1 nm
Al.sub.2O.sub.3 film in the "thin Cr field." A small effect is
noted at the edges of Cr features, but no effect within the Cr
features was noticed. Over-layered images of irradiated and
unirradiated sites on the sample show no critical dimension (CD)
change.
FIG. 13 is a graph depicting optical transmission and reflectivity
vs. wavelength, as a function of wavelength of fused silica with
and without 5 nm of an ALD Al.sub.2O.sub.3 coating.
FIG. 14 is a graph depicting reflectivity as a function of
wavelength of fused silica with and without 5 nm of an ALD
Al.sub.2O.sub.3 coating.
FIG. 15 is a graph depicting change in optical transmission due to
5 nm ALD Al.sub.2O.sub.3 coating on fused silica mask, i.e.
plotting transmission difference corrected for reflectivity
difference, as a function of wavelength of an ALD Al.sub.2O.sub.3
coating on said fused silica mask reflecting a .DELTA.T.
ANALYSIS OF THE SEM PHOTOS
The samples with 5 nm ALD deposited Al.sub.2O.sub.3 coating showed
no degradation as a result of being exposed to 193 nm irradiation;
likewise, the samples containing the 1.1 nm ALD deposited
Al.sub.2O.sub.3 coating showed no degradation as a result of being
exposed to 193 nm irradiation.
An exception to the "no degradation result" was that with respect
to the COG sample which was oxygen ashed after deposition and then
irradiated, 1.1 nm ALD Al.sub.2O.sub.3 increased optical absorption
at 193 nm by less than 0.003%.
In summary, an inspection of the SEM photos in FIGS. 5-12 and the
data presented in FIGS. 13-15, supports the conclusion that the
molybdenum silicide and chromium-based photomasks treated with an
atomic layer deposition (ALD) Al.sub.2O.sub.3 coating do not
degrade when exposed to 193 nm UV wavelength as compared with
molybdenum silicide and chromium-based photomasks that do not have
this ALD Al.sub.2O.sub.3 coating on their respective surfaces.
The deposition of a thin film of Al.sub.2O.sub.3 using atomic layer
deposition protects both MoSi and Chrome on Glass photomasks from
degradation.
The present invention contemplates implementation of the deposition
of the ALD Al.sub.2O.sub.3 coating on said photomask using a system
or systems that provide multi-processor, multi-tasking,
multi-process, and/or multi-thread computing, as well as
implementation on systems that provide only single processor,
single thread computing.
Multi-processor computing involves performing computing using more
than one processor. Multi-tasking computing involves performing
computing using more than one operating system task.
A task is an operating system concept that refers to the
combination of a program being executed and bookkeeping information
used by the operating system. Whenever a program is executed, the
operating system creates a new task for it.
The task is like an envelope for the program in that it identifies
the program with a task number and attaches other bookkeeping
information to it. Many operating systems, including Linux,
UNIX.RTM., OS/2.RTM., and Windows.RTM., are capable of running many
tasks at the same time and are called multitasking operating
systems.
Multi-tasking is the ability of an operating system to execute more
than one executable at the same time. Each executable is running in
its own address space, meaning that the executables have no way to
share any of their memory. This has advantages, because it is
impossible for any program to damage the execution of any of the
other programs running on the system. However, the programs have no
way to exchange any information except through the operating system
(or by reading files stored on the file system).
Multi-process computing is similar to multi-tasking computing, as
the terms task and process are often used interchangeably, although
some operating systems make a distinction between the two.
The present invention may be a system, a method, and/or a computer
program product at any possible technical detail level of
integration. The computer program product may include a computer
readable storage medium (or media) having computer readable program
instructions thereon for causing a processor to carry out aspects
of the present invention.
The computer readable storage medium as mentioned above, can be a
tangible photolithographic device that can retain and store
instructions for use by an instruction execution device. The
computer readable storage medium is an optical storage device, but
may also be an electronic storage device, a magnetic storage
device, an electromagnetic storage device, a semiconductor storage
device, or any suitable combination of the foregoing. A
non-exhaustive list of more specific examples of the computer
readable storage medium includes the following: a portable computer
diskette, a hard disk, a random access memory (RAM), a read-only
memory (ROM), an erasable programmable read-only memory (EPROM or
Flash memory), a static random access memory (SRAM), a portable
compact disc read-only memory (CD-ROM), a digital versatile disk
(DVD), a memory stick, a floppy disk, a mechanically encoded device
such as punch-cards or raised structures in a groove having
instructions recorded thereon, and any suitable combination of the
foregoing.
A computer readable storage medium, as used herein, is not to be
construed as being transitory signals per se, such as radio waves
or other freely propagating electromagnetic waves, electromagnetic
waves propagating through a waveguide or other transmission media
(e.g., light pulses passing through a fiber-optic cable), or
electrical signals transmitted through a wire.
Computer readable program instructions described herein can be
downloaded to respective computing/processing devices from a
computer readable storage medium or to an external computer or
external storage device via a network, for example, the Internet, a
local area network, a wide area network and/or a wireless network.
The network may comprise copper transmission cables, optical
transmission fibers, wireless transmission, routers, firewalls,
switches, gateway computers, and/or edge servers.
A network adapter card or network interface in each
computing/processing device receives computer readable program
instructions from the network and forwards the computer readable
program instructions for storage in a computer readable storage
medium within the respective computing/processing device.
Computer readable program instructions for carrying out operations
of the present invention may be assembler instructions,
instruction-set-architecture (ISA) instructions, machine
instructions, machine dependent instructions, microcode, firmware
instructions, state-setting data, configuration data for integrated
circuitry, or either source code or object code written in any
combination of one or more programming languages, including an
object oriented programming language such as Smalltalk, C++, or the
like, and procedural programming languages, such as the "C"
programming language or similar programming languages. The computer
readable program instructions may execute entirely on the user's
computer, partly on the user's computer, as a stand-alone software
package, partly on the user's computer and partly on a remote
computer or entirely on the remote computer or server. In the
latter scenario, the remote computer may be connected to the user's
computer through any type of network, including a local area
network (LAN) or a wide area network (WAN), or the connection may
be made to an external computer (for example, through the Internet
using an Internet Service Provider). m
In some embodiments, electronic circuitry including, for example,
programmable logic circuitry, field-programmable gate arrays
(FPGA), or programmable logic arrays (PLA) may execute the computer
readable program instructions by utilizing state information of the
computer readable program instructions to personalize the
electronic circuitry, in order to perform aspects of the present
invention.
These computer readable program instructions may be provided to a
processor of a general-purpose computer, special purpose computer,
or other programmable data processing apparatus to produce a
machine, such that the instructions, which execute via the
processor of the computer or other programmable data processing
apparatus, create means for implementing the functions/acts
specified in the description provided above.
The aforementioned computer readable program instructions may also
be stored in a computer readable storage medium that can direct a
computer, a programmable data processing apparatus, and/or other
devices to function in a particular manner, such that the computer
readable storage medium having instructions stored therein
comprises an article of manufacture including instructions which
implement aspects of the function/act specified in the flowchart
and/or block diagram block or blocks.
The computer readable program instructions may also be loaded onto
a computer, other programmable data processing apparatus, or other
device to cause a series of operational steps to be performed on
the computer, other programmable apparatus or other device to
produce a computer implemented process, such that the instructions
which execute on the computer, other programmable apparatus, or
other device implement the functions/acts specified in the
flowchart and/or block diagram block or blocks.
For example, two blocks shown in succession may, in fact, be
executed substantially concurrently, or the blocks may sometimes be
executed in the reverse order, depending upon the functionality
involved.
Although specific embodiments of the present invention have been
described, it will be understood by those of skill in the art that
there are other embodiments that are equivalent to the described
embodiments. Accordingly, it is to be understood that the invention
is not to be limited by the specific illustrated embodiments, but
only by the scope of the appended claims.
* * * * *
References